The present invention relates to a method and apparatus for representing gray scale values in facsimile transmission of material for printing on a recording medium, and specifically relates to such a method wherein an original having graphic patterns thereon is scanned along several parallel lines and gray scale value signals associated with the gray scale values of the graphic pattern are generated, which gray scale values are represented by means of individual dots of equal size and the number of such dots per unit area corresponds to each gray scale value.
It is known to employ writing stations provided with a mosaic printing head not only for representing alphanumeric characters but also for receiving information as part of a facsimile transmission. The characters and the graphic pattern in such writing stations are represented by means of dots of equal size which are generated by the mosaic printing head. Because generation dots with varied sizes is not possible in the case of most mosaic printing heads, gray scale values must be displayed on a recording medium as pseudo half tones comprised of dots of the same size. In order to achieve a number of such pseudo half tones, a specific number of dots per unit area is associated with each gray scale value.
It is possible to generate such pseudo half tones by providing an adder stage which sums instantaneous values of gray scale value signals which are generated during a scanning of an original along one line. When the sum exceeds a predetermined threshold value, a print command may be released to the mosaic printing head. The sum is simultaneously decreased by the threshold value. If a uniformly gray area is scanned on a plurality of lines, there is the possibility that, in the case of equal initial values, the sums associated with all lines will simultaneously reach the threshold value and the uniformly gray area will thus be depicted by means of dashes which are perpendicular to the scanned lines. Such vertical dashes can result in moire patterns and moreover, in such a representation, it cannot be determined whether a uniformly gray area on the original or an area on the original having vertical dashes was scanned.